| Engineering Journal 5.4.4: The timers basic and no metric holddown Commands Following are descriptions of the timers basic and no metric holddown commands used to help you control routing updates. The timers basic Command The timers basic command lets you control how often IGRP sends updates. The default is once every 90 seconds. In order to allow for dropped packets, IGRP can't time out expired routes until several minutes have elapsed. When IGRP removes a route, it can't adopt a new one for several more minutes because of holddown. The first thing to do is to speed up the time constants. Use 15 seconds instead of 90 for the basic time constant. This allows routes to expire after 45 seconds. All the other times decrease proportionally. Actually, the expiration time turns out not to be as important as you might expect. Normally, routes don't just expire. They are killed because a keepalive fails on some interface. Keepalives normally occur every 10 seconds, so it takes 30 seconds to detect an interface down that way. You should use a keepalive of 4 on T1 lines where you care about speed of routing adjustment. This lets you detect a failure in 12 seconds. The no metric holddown Command The other critical parameter to change in order to accept a new route is no metric holddown. This disables holddowns, meaning that after a route has been removed, a new one is accepted immediately. There are good theoretical reasons for using holddowns. For example, there could be cases where, without holddowns, an old route can never get out of the system.